Small Craft Advisory 470, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
542 AM AKDT Mon Jun 8 2020

.ANALYSIS AND UPPER LEVELS...

The slow change in the weather pattern continues today. At 500MB,
the longwave pattern still has troughing in the northern Gulf
stretching west to southwest AK. There are two closed lows within
this trough, one diving southeast into the North Pacific and the
other slowly spinning west along the north Gulf coast. The movement
of both of these lows is finally driving change in the weather.

At the surface, the low in the Central Bering continues diving to
the southeast into the North Pacific, with the backside of the
low now over the Aleutian Island chain. It is supported by the low
at 500MB in the same area. As this system exits, low ceilings and
fog are creeping back into Southwest Alaska. Meanwhile, a weak
low in the Copper River Basin is sending weather into the Susitna
Valley and Anchorage Bowl, bringing cloud cover and light rain.
It is also removing the threat of the convection in Anchorage
Bowl, which we experienced yesterday afternoon and evening. So
overall, the slow change carries on for our forecast area.

&&

.MODEL DISCUSSION...

Models continue to struggle past about 36 hours. All of the
models capture the major features, but there is significant
disagreement among them. By Wednesday, all models have a large
low in the western Bering and another low just south of the Gulf
of Alaska. However, looking at a surface chart at that time shows
disagreement in the placement of the low centers for both systems.
Additionally, the GFS has the low in the western Bering a full
10MB deeper than any other model. This is quite a disparity.
Given that there is almost no agreement on the major systems, we
can only say where the general inclement weather will be. Details
are hard to come by with this level of disagreement. Therefore,
confidence is low in the models by midweek.

&&

.AVIATION...
PANC...VFR conditions will persist. Turnagain arm winds will bend
into the airfield this afternoon and evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Monday
and Tuesday)...

Looking at a rainy start to the work week as an easterly shortwave
makes its way from the Copper River Basin this morning and down
across Kodiak Island by Tuesday night. At the surface a weak
coastal ridge is expected to bring some gusty winds into the
Turnagain Arm and Copper River Basin through midnight tonight.
Radar and surface observations indicate increasingly widespread
precipitation over the Prince William Sound, Mat-Su Valley, and
Copper River Basin this morning. This rain is expected to reach
the Kenai Peninsula midday today and arrive at Kodiak Island early
Tuesday morning. Extensive cloud cover and statically stable air
should keep thunderstorms at bay today today although there is a
slight possibility of a thunderstorm or two over the higher
elevations of the western Alaska Range. Rainfall should begin to
slowly taper off Tuesday night over most of southcentral, with
some lingering showers over the higher elevations and Kodiak Island.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

The infamous thermal trough from the last couple days combined
with diurnal heating will once again trigger scattered showers
and isolated thunderstorms today beginning around mid-morning
through the evening hours. As the thermal trough axis shifts
eastward in the next day or two, showers and thunderstorms will
exit the Kuskokwim Delta and western Kuskokwim Valley and set up
shop along the west side of the Alaska Range. Meanwhile, an upper
level trough will drop down from the Arctic and along the west
coast of Alaska, ushering in drier more stable conditions to the
Kuskokwim Delta and inland to portions of the Kuskokwim Valley
tonight through Tuesday night. Onshore flow off the Bering Sea and
Bristol Bay also means a threat of low clouds and fog each night
and morning.

Farther east over the Gulf of Alaska and Southcentral Alaska, a
series of shortwaves will rotate westward, likely making it far
enough west to impact eastern-most Southwest Alaska (near the
Alaska and Aleutian Ranges). At a minimum, this will enhance
convection over the next couple days. However, there is also
potential for some areas of steady rain with these, most likely
overnight tonight through Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A fast-moving low pressure system south of Dutch Harbor producing
small craft to gale force winds across the eastern Aleutians this
morning will quickly exit across the north Pacific today, leading
to improving conditions. However, a new storm over the northwest
Pacific will send a front toward the Aleutian chain today. A
second stronger low will then develop and lead to widespread rain
and small craft to gale force winds across the Aleutians and
southern Bering Sea Tuesday through Wednesday. There is some
uncertainty with the track and strength of this low, but computer
models all now indicate a deeper low, so the forecast has been
trended toward stronger winds.

&&

.MARINE (Days 3 through 5...Wed through Fri)...

A small swath of gales will move across the western and central
Aleutians on Wednesday before the low weakens with winds becoming
small craft.

For the Gulf of Alaska, winds and seas are expected to remain
relatively light and calm (below Small Craft).

&&

.LONG TERM FORECAST (Days 3 through 7...Wed through Sun)...

Wednesday, ridging will build over Southcentral bringing an
increase to temperatures and a drying trend across the area.
Convection potential will remain due to diurnal heating associated
with this feature. This looks to remain until Saturday when an
upper level wave from a low centered over the Bering Sea will
propagate into southern Alaska. This will bring an increase in
precipitation to Kodiak Island and the Kenai Peninsula. Sunday, an
increase in precipitation will return to Southcentral due to an
easterly wave moving into the area from Canada, however,
confidence of this is low.

For the Aleutians, an area of low pressure will bring an increase
to precipitation and winds to the Bering as it moves towards the
Alaska Peninsula through Sunday. Beginning Saturday, another front
will move across the western Aleutians bringing more chances for
rain and increased winds to the area through Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES: 172, 177, 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BJB
SOUTHCENTRAL ALASKA...CJ/TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB/SB
MARINE/LONG TERM...CK